The thalamic input to the visual cortex of the rat will be studied in the electron microscope. An attempt will be made to determine the source of the dendritic spines upon which 95 per cent of degenerating thalamic axons terminate To do this the parent dendrites giving rise to the spines have to be recognized. This is difficult because in random thin sections most of the spine profiles are not seen to be connected to the parent dendrites. It is hoped to establish the form of the parent dendrites by making reconstructions based upon serial sections. Since some of the spines could arise from spiney stellate cells the form and dstribution of such neurons will be determined using Golgi impregnated material. A study of the late prenatal rat cortex will be continued to allow a description to be given of the intermediate and ventricular layers.